In oilfield jargon, “plug and abandon” or “P&A” refers to preparing a well to be closed permanently (or at least until prices or technology developments warrant reentry). The decision to plug and abandon a well or field is an economic decision. Once production value drops below operating expenses, it is time to consider abandonment, even if considerable reserves remain. Thus, well abandonment is an inevitable stage in the lifespan of a well.
The earliest oil wells were abandoned without any plugging, and the first plugging requirements were enacted by Pennsylvania in the 1890s. Many wells were abandoned with plugs consisting of brush, wood, paper sacks, linen or any other material that could be pushed into a well to form a basis for the dumping of one or two sacks of cement to “plug” the well.
Current plugging procedures are significantly more disciplined due to modern regulations. The P&A regulations vary among states and between countries, but all regulations prescribe the depth intervals that must be cemented, as well as the materials that are allowed in plugging practices. Most states require that cement plugs be placed and tested across any open hydrocarbon-bearing formations, across all casing shoes, across freshwater aquifers, and perhaps several other areas near the surface, including the top 20 to 50 feet [6 to 15 m] of the wellbore. Some countries also require that a “rock-to-rock” cement plug be set that is contact with wellbore outside the casing if the casing is not isolated with cement.
In recognition of its strength, low permeability and low cost, cement is typically used to create a seal between formations or to seal off the surface of the wellbore. Other materials that do not offer the same strength or durability as cement, including drilling mud, gel, and clay, are used to fill in the spaces between cement plugs. Additionally, many states allow the use of mechanical bridge plugs in lieu of a large cement plug since the bridge plug is extremely strong and nearly completely impermeable. However, mechanical plugs are susceptible to corrosion and elastomer failure, and therefore the regulations typically require the bridge plugs to be capped by a specified amount of cement.
One of the main problems in any cementing procedure is contamination of the cement, leading to early failure. Poor mud-removal in the area where the cement is to be set can give rise to channels through the plug caused by the drilling fluid. To avoid this, a spacer is often pumped before and after the cement slurry to wash the hole and to segregate the drilling fluid and the cement from each other. However, issues can still arise.
Because cement is susceptible to early failure if contaminated by drilling or other fluids, other materials have been investigated for use as plugging material. Resins offer superior adhesion, resistance to many caustic and corrosive chemicals, excellent mechanical properties such as low yield point and low viscosity in the unset state, and flexibility and toughness after setting. Resin sealing materials include ThermaSet® by Wellcem AS, CannSeal® by AGR, and the WellLock® resin system by Halliburton. M&D Industries also makes resin plugging materials, including LIQUID BRIDGE PLUG® with a range of hardeners and accelerators. The WellLock® resin, for example, uses cross-linking between an amine hardener and epoxides, resulting in a cured three-dimensional infinite polymer network, and can be deployed without negative impact from exothermic reactions triggered by water.
New types of cement slurries consisting of geopolymeric materials have also been developed as alternative to the conventional lightweight cement slurry. Geopolymers are made of aluminum and silicon and they exhibit superior mechanical and chemical properties compared to the Class G cement. Geopolymers can provide a material with specific properties from a range of cement/flyash/aluminiosilicate component ratios. This gives a light-weight slurry with high compressive and flexural strength thought to replace the conventional lightweight cements containing silica fume.
Sandaband is another cement alternative. It is a sand-slurry consisting of about three quarters sand particles and one quarter water and other additives, developed in Norway to meet the increasing demands of an everlasting plugging material. Sandaband possesses the properties as a Bingham fluid and acts as a deformable solid when it's stationary, but as a liquid when in motion. This ductile behavior means that the sand slurry will never fracture or create micro annuli. The sand slurry is also incompressible and gas tight, and does not shrink, fracture or segregate. It does however require a solid foundation, as it will sink if placed on another fluid.
As noted above, many regulations now also required that the plug be set across the entire borehole. As most wells have at least one casing string or liner, access to the annular space to set the plug is problematic.
Currently, operators have to remove sections of casing so that a plug may be set that is continuous across the entire borehole in a configuration often referred to as “rock-to-rock,” and located in the cap rock above the reservoir. Because cement or other plugging material must go all the way to the formation wall, the typical procedure was to pull the tubing, mill the casing, and remove swarf before spotting the cement (FIG. 1). However, this process may require multiple trips downhole and the tons of swarf that must be removed can accumulate in low flow zones, and has razor sharp edges, being hazardous to both drill crew and equipment. Plus, the method is expensive and time-consuming, and it has not been successfully applied to reservoirs with more than two strings.
One response to these challenges has been the introduction of a system known as perforate, wash and cement (PWC) in a single run. The PWC operation is designed to access the formation through perforations in the casing to place a rock-to-rock cement barrier without removing the casing, thus saving valuable rig time. To use this system, the well must be secured, Christmas tree removed, tubing pulled, and then PWC job can be done.
The PWC method uses a special tool by Archer, described in US20150053405. The tool is made of pipe conveyed perforating guns attached below a wash tool, which is below a cement stinger. Using PWC, ConocoPhillips completed 20 PWC plug installations in the North Sea, reducing the time required to set a permanent plug to 2.6 days from 10.5 days using section milling. As a result, the company calculated a savings of 124 rig days over the course of the 20 PWC wells. Given that rig time can easily be upwards of half a million dollars per day for an offshore rig, even a few days less time required for P&A can mean significant cost savings.
Although an improvement, the PWC method has limitations. To date, the PWC method has not been successfully applied through multiple casings. Furthermore, it is difficult to implement this method if the pipe has deformed such that the lengthy tool can no longer pass through the deviated section. Another disadvantage of both the milling and PWC methods is that the well must be secure and the tubing has to be pulled to implement these methods, which raises costs.
Thus, what is needed in the art are better methods, devices and systems for P&A that are safe, create a reliable barrier, that are cost effective, and both faster and easier to perform than current methods. Ideally, the new method would be performed through tubing, and could provide access to the annular space to allow for a “rock-to-rock” plug. An ideal system would also be applicable to wellbores with two or more casings or strings.